If the Atlanta Braves could have one mulligan from this past offseason, it probably would be their decision to non-tender second baseman Kelly Johnson. While the Braves have struggled to hit for power, Johnson has supplied it in droves for the Arizona Diamondbacks this season. He has 10 home runs already, which is as many as all Braves not named Jason Heyward have hit. But although Johnson's torrid start has been impressive, it's unlikely he'll remain this good throughout 2010.
Johnson first showed signs of his potential in 2007, when he hit .276/.375/.457 (batting average/on-base percentage/slugging percentage) as the Braves' full-time second baseman. He struggled in the next two years, however, hence the non-tender. In many ways, the start to his 2010 season resembles that breakout season. His walk and strikeout rates are back in line. His batted ball data also line up fairly well, although Johnson is hitting a few more fly balls in 2010. Still, the results on those fly balls have been much better. Of the 35 fly balls he's hit, 10 have left the park (28.6 percent). That mark is almost certainly unsustainable during the course of a full season and is higher than any hitter's rate last year.
In 2009, only seven NL hitters cleared the fence with more than 20 percent of their fly balls. Mark Reynolds led the way with 26 percent. (See list below.) One thing that stands out about the names you see is that they all have high career isolated power (ISO) numbers, a statistic that is calculated by subtracting batting average from slugging percentage. Among last year's leaders in home run rate, only Raul Ibanez doesn't have a career ISO of .200 or higher, and he's close at .194. Johnson has a career ISO of .178, so he is a prime candidate to regress.
Pop but no power
Ibanez has a low career isolated power, which hinted at his regression.
Ibanez actually provides a cautionary tale of his own. Before his mid-June injury last season, he hit 22 home runs on 85 fly balls, a 25.9 percent rate, while his career rate is just 13 percent. From his mid-July return through the end of the season, he hit 75 fly balls, only 12 of which left the park (16 percent). That percentage still ranks above his career mark but certainly brought down the numbers he produced during his powerful start.
Even when Johnson does come back to Earth, he'll likely continue to produce for the Diamondbacks. Sabermetrician Dan Szymborski's ZiPS projection, which he updates daily, has Johnson pegged for a .219 ISO the rest of the way, so his power should remain intact even when more of his fly balls start dropping into gloves or gaps. He won't sustain his .355 ISO for the next five months, but a .251 mark, per ZiPS, would represent a huge step forward for Johnson.
Joe Pawlikowski is a writer for FanGraphs.